The present invention is directed to a process for preparing a polyisocyanate by reacting the corresponding primary amine with phosgene in an adiabatically managed reaction.
The large-scale preparation of polyisocyanates by reacting amines with phosgene in solvents is well-known and is described in detail in the literature.
DE-A-34 03 204, for example, describes a continuous process for preparing organic polyisocyanates in which an elevated temperature of 100 to 220° C. is used at a pressure of 5-100 bar in a reaction that is performed in a circulated system to some extent.
DE-A-17 68 439 describes a process for the continuous preparation of organic isocyanates in which the amine and phosgene are first preheated and then the preheated constituents are brought together in the reaction zone under high pressure and are reacted under isothermal conditions, i.e. while undergoing heat exchange with the surroundings.
DE-A-102 22 968 describes a process for the continuous preparation of polyisocyanates by reacting primary amines with phosgene in which the reaction is performed in a cascade of temperature-controlled reaction tubes of different sizes.
A common feature of these processes is that, in order to adjust to the desired reaction temperature, temperature-controlled reactors of one form or another (jacket heating, heating via heat-exchangers or special reactor inserts) are unavoidable. However, in particular when synthesizing isocyanates by the phosgenation of amines, external temperature control of the reactors is often a problem because the high temperatures at the walls of the reactor promote or cause the formation of secondary products which then have a negative effect on the yield and/or the properties of the product. In addition, deposits are formed in the reactor which means that the reactors have to be switched off and cleaned at regular intervals. However, this leads to the loss of plant capacity and thus to an economic disadvantage. In addition, the heat transfer units require additional investment costs, which also adversely affect the economic viability of the process.